Angina is chest pain or discomfort due to coronary heart disease. It occurs when the heart muscle doesn’t get as much blood as it needs. This usually happens because one or more of the heart’s arteries is narrowed or blocked, also called ischemia. Stable angina refers to “predictable” chest discomfort associated with exertion or stress. Unstable angina refers to unexpected chest pain and usually occurs at rest. It is typically more severe and prolonged. Unstable angina should be treated as an emergency.